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The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	1	<html>
				2	<head>
				3	<title>Android JNI Tips</title>
				4	<link rel=stylesheet href="android.css">
				5	</head>
				6
				7	<body>
				8	<h1><a name="JNI_Tips"></a>Android JNI Tips</h1>
				9	<p>
				10	</p><p>
				11	</p><ul>
				12	<li> <a href="#What_s_JNI_">What's JNI?</a>
				13	</li>
				14	<li> <a href="#JavaVM_and_JNIEnv">JavaVM and JNIEnv</a>
				15
				16	</li>
				17	<li> <a href="#jclassID_jmethodID_and_jfieldID">jclassID, jmethodID, and jfieldID</a>
				18	</li>
				19	<li> <a href="#local_vs_global_references">Local vs. Global References</a>
				20	</li>
				21	<li> <a href="#UTF_8_and_UTF_16_strings">UTF-8 and UTF-16 Strings</a>
				22	</li>
				23	<li> <a href="#Arrays">Primitive Arrays</a>
				24	</li>
				25	<li> <a href="#RegionCalls">Region Calls</a>
				26	</li>
				27	<li> <a href="#Exceptions">Exceptions</a>
				28	</li>
				29
				30	<li> <a href="#Extended_checking">Extended Checking</a>
				31	</li>
				32	<li> <a href="#Native_Libraries">Native Libraries</a>
				33	</li>
				34	<li> <a href="#64bit">64-bit Considerations</a>
				35	</li>
				36
				37	<li> <a href="#Unsupported">Unsupported Features</a>
				38	</ul>
				39	<p>
				40	<noautolink>
				41	</noautolink></p><p>
				42	</p><h2><a name="What_s_JNI_"> </a> What's JNI? </h2>
				43	<p>
				44
				45	JNI is the Java Native Interface. It defines a way for code written in the
				46	Java programming language to interact with native
				47	code, e.g. functions written in C/C++. It's VM-neutral, has support for loading code from
				48	dynamic shared libraries, and while cumbersome at times is reasonably efficient.
				49	</p><p>
				50	You really should read through the
				51	<a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/jniTOC.html">JNI spec for J2SE 1.6</a>
				52	to get a sense for how JNI works and what features are available. Some
				53	aspects of the interface aren't immediately obvious on
				54	first reading, so you may find the next few sections handy.
				55	The more detailed <i>JNI Programmer's Guide and Specification</i> can be found
				56	<a href="http://java.sun.com/docs/books/jni/html/jniTOC.html">here</a>.
				57	</p><p>
				58	</p><p>
				59	</p><h2><a name="JavaVM_and_JNIEnv"> </a> JavaVM and JNIEnv </h2>
				60	<p>
				61	JNI defines two key data structures, "JavaVM" and "JNIEnv". Both of these are essentially
				62	pointers to pointers to function tables. (In the C++ version, it's a class whose sole member
				63	is a pointer to a function table.) The JavaVM provides the "invocation interface" functions,
				64	which allow you to create and destroy the VM. In theory you can have multiple VMs per process,
				65	but Android's VMs only allow one.
				66	</p><p>
				67	The JNIEnv provides most of the JNI functions. Your native functions all receive a JNIEnv as
				68	the first argument.
				69	</p><p>
				70
				71	On some VMs, the JNIEnv is used for thread-local storage. For this reason, <strong>you cannot share a JNIEnv between threads</strong>.
				72	If a piece of code has no other way to get its JNIEnv, you should share
				73	the JavaVM, and use JavaVM->GetEnv to discover the thread's JNIEnv.
				74	</p><p>
				75	The C and C++ declarations of JNIEnv and JavaVM are different. "jni.h" provides different typedefs
				76	depending on whether it's included into ".c" or ".cpp". For this reason it's a bad idea to
				77	include JNIEnv arguments in header files included by both languages. (Put another way: if your
				78	header file requires "#ifdef __cplusplus", you may have to do some extra work if anything in
				79	that header refers to JNIEnv.)
				80	</p><p>
				81	</p><p>
				82	</p><h2><a name="jclassID_jmethodID_and_jfieldID"> jclassID, jmethodID, and jfieldID </a></h2>
				83	<p>
				84	If you want to access an object's field from native code, you would do the following:
				85	</p><p>
				86	</p><ul>
				87	<li> Get the class object reference for the class with <code>FindClass</code>
				88	</li>
				89	<li> Get the field ID for the field with <code>GetFieldID</code>
				90	</li>
				91	<li> Get the contents of the field with something appropriate, e.g.
				92	<code>GetIntField</code>
				93	</li>
				94	</ul>
				95	<p>
				96	Similarly, to call a method, you'd first get a class object reference and then a method ID. The IDs are often just
				97	pointers to internal VM data structures. Looking them up may require several string
				98	comparisons, but once you have them the actual call to get the field or invoke the method
				99	is very quick.
				100	</p><p>
				101	If performance is important, it's useful to look the values up once and cache the results
				102	in your native code. Because we are limiting ourselves to one VM per process, it's reasonable
				103	to store this data in a static local structure.
				104	</p><p>
				105	The class references, field IDs, and method IDs are guaranteed valid until the class is unloaded. Classes
				106	are only unloaded if all classes associated with a ClassLoader can be garbage collected,
				107	which is rare but will not be impossible in our system. The jclassID
				108	is a class reference and <strong>must be protected</strong> with a call
				109	to <code>NewGlobalRef</code> (see the next section).
				110	</p><p>
				111	If you would like to cache the IDs when a class is loaded, and automatically re-cache them
				112	if the class is ever unloaded and reloaded, the correct way to initialize
				113	the IDs is to add a piece of code that looks like this to the appropriate class:
				114	</p><p>
				115
				116	</p><pre> /*
				117	* We use a class initializer to allow the native code to cache some
				118	* field offsets.
				119	*/
				120
				121	/*
				122	* A native function that looks up and caches interesting
				123	* class/field/method IDs for this class. Returns false on failure.
				124	*/
				125	native private static boolean nativeClassInit();
				126
				127	/*
				128	* Invoke the native initializer when the class is loaded.
				129	*/
				130	static {
				131	if (!nativeClassInit())
				132	throw new RuntimeException("native init failed");
				133	}
				134	</pre>
				135	<p>
				136	Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code
				137	will be executed once, when the class is initialized. If the class is ever unloaded and
				138	then reloaded, it will be executed again. (See the implementation of java.io.FileDescriptor
				139	for an example in our source tree.)
				140	</p><p>
				141	</p><p>
				142	</p><p>
				143	</p><h2><a name="local_vs_global_references"> Local vs. Global References </a></h2>
				144	<p>
				145	Every object that JNI returns is a "local reference". This means that it's valid for the
				146	duration of the current native method in the current thread.
				147	<strong>Even if the object itself continues to live on after the native method returns, the reference is not valid.</strong>
				148	This applies to all sub-classes of jobject, including jclass and jarray.
				149	(Dalvik VM will warn you about this when -Xcheck:jni is enabled.)
				150	</p><p>
				151
				152	If you want to hold on to a reference for a longer period, you must use a "global" reference.
				153	The <code>NewGlobalRef</code> function takes the local reference as
				154	an argument and returns a global one:
				155
				156	<p><pre>jobject* localRef = [...];
				157	jobject* globalRef;
				158	globalRef = env->NewGlobalRef(localRef);
				159	</pre>
				160
				161	The global reference is guaranteed to be valid until you call
				162	<code>DeleteGlobalRef</code>.
				163	</p><p>
				164	All JNI methods accept both local and global references as arguments.
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	165	It's possible for references to the same object to have different values;
				166	for example, the return values from consecutive calls to
				167	<code>NewGlobalRef</code> on the same object may be different.
				168	<strong>To see if two references refer to the same object,
				169	you must use the <code>IsSameObject</code> function.</strong> Never compare
				170	references with "==" in native code.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	171	</p><p>
				172	Programmers are required to "not excessively allocate" local references. In practical terms this means
				173	that if you're creating large numbers of local references, perhaps while running through an array of
				174	Objects, you should free them manually with
				175	<code>DeleteLocalRef</code> instead of letting JNI do it for you. The
				176	VM is only required to reserve slots for
				177	16 local references, so if you need more than that you should either delete as you go or use
				178	<code>EnsureLocalCapacity</code> to reserve more.
				179	</p><p>
				180	Note: method and field IDs are just 32-bit identifiers, not object
				181	references, and should not be passed to <code>NewGlobalRef</code>. The raw data
				182	pointers returned by functions like <code>GetStringUTFChars</code>
				183	and <code>GetByteArrayElements</code> are also not objects.
				184	</p><p>
				185	One unusual case deserves separate mention. If you attach a native
				186	thread to the VM with AttachCurrentThread, the code you are running will
				187	never "return" to the VM until the thread detaches from the VM. Any local
				188	references you create will have to be deleted manually unless the thread
				189	is about to exit or detach.
				190	</p><p>
				191	</p><p>
				192	</p><p>
				193	</p><h2><a name="UTF_8_and_UTF_16_strings"> </a> UTF-8 and UTF-16 Strings </h2>
				194	<p>
				195	The Java programming language uses UTF-16. For convenience, JNI provides methods that work with "modified UTF-8" encoding
				196	as well. (Some VMs use the modified UTF-8 internally to store strings; ours do not.) The
				197	modified encoding only supports the 8- and 16-bit forms, and stores ASCII NUL values in a 16-bit encoding.
				198	The nice thing about it is that you can count on having C-style zero-terminated strings,
				199	suitable for use with standard libc string functions. The down side is that you cannot pass
				200	arbitrary UTF-8 data into the VM and expect it to work correctly.
				201	</p><p>
				202	It's usually best to operate with UTF-16 strings. With our current VMs, the
				203	<code>GetStringChars</code> method
				204	does not require a copy, whereas <code>GetStringUTFChars</code> requires a malloc and a UTF conversion. Note that
				205	<strong>UTF-16 strings are not zero-terminated</strong>, and \u0000 is allowed,
				206	so you need to hang on to the string length as well as
				207	the string pointer.
				208
				209	</p><p>
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	210	<strong>Don't forget to Release the strings you Get</strong>. The
				211	string functions return <code>jchar</code> or <code>jbyte</code>, which
				212	are C-style pointers to primitive data rather than local references. They
				213	are guaranteed valid until Release is called, which means they are not
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	214	released when the native method returns.
				215	</p><p>
				216	</p><p>
				217
				218
				219	</p><h2><a name="Arrays"> </a> Primitive Arrays </h2>
				220	<p>
				221	JNI provides functions for accessing the contents of array objects.
				222	While arrays of objects must be accessed one entry at a time, arrays of
				223	primitives can be read and written directly as if they were declared in C.
				224	</p><p>
				225	To make the interface as efficient as possible without constraining
				226	the VM implementation,
				227	the <code>Get<PrimitiveType>ArrayElements</code> family of calls
				228	allows the VM to either return a pointer to the actual elements, or
				229	allocate some memory and make a copy. Either way, the raw pointer returned
				230	is guaranteed to be valid until the corresponding <code>Release</code> call
				231	is issued (which implies that, if the data wasn't copied, the array object
				232	will be pinned down and can't be relocated as part of compacting the heap).
				233	<strong>You must Release every array you Get.</strong> Also, if the Get
				234	call fails, you must ensure that your code doesn't try to Release a NULL
				235	pointer later.
				236	</p><p>
				237	You can determine whether or not the data was copied by passing in a
				238	non-NULL pointer for the <code>isCopy</code> argument. This is rarely
				239	useful.
				240	</p><p>
				241	The <code>Release</code> call takes a <code>mode</code> argument that can
				242	have one of three values. The actions performed by the VM depend upon
				243	whether it returned a pointer to the actual data or a copy of it:
				244	<ul>
				245	<li><code>0</code>
				246	<ul>
				247	<li>Actual: the array object is un-pinned.
				248	<li>Copy: data is copied back. The buffer with the copy is freed.
				249	</ul>
				250	<li><code>JNI_COMMIT</code>
				251	<ul>
				252	<li>Actual: does nothing.
				253	<li>Copy: data is copied back. The buffer with the copy
				254	<strong>is not freed</strong>.
				255	</ul>
				256	<li><code>JNI_ABORT</code>
				257	<ul>
				258	<li>Actual: the array object is un-pinned. Earlier
				259	writes are <strong>not</strong> aborted.
				260	<li>Copy: the buffer with the copy is freed; any changes to it are lost.
				261	</ul>
				262	</ul>
				263	</p><p>
				264	One reason for checking the <code>isCopy</code> flag is to know if
				265	you need to call <code>Release</code> with <code>JNI_COMMIT</code>
				266	after making changes to an array -- if you're alternating between making
				267	changes and executing code that uses the contents of the array, you may be
				268	able to
				269	skip the no-op commit. Another possible reason for checking the flag is for
				270	efficient handling of <code>JNI_ABORT</code>. For example, you might want
				271	to get an array, modify it in place, pass pieces to other functions, and
				272	then discard the changes. If you know that JNI is making a new copy for
				273	you, there's no need to create another "editable" copy. If JNI is passing
				274	you the original, then you do need to make your own copy.
				275	</p><p>
				276	Some have asserted that you can skip the <code>Release</code> call if
				277	<code>*isCopy</code> is false. This is not the case. If no copy buffer was
				278	allocated, then the original memory must be pinned down and can't be moved by
				279	the garbage collector.
				280	</p><p>
				281	Also note that the <code>JNI_COMMIT</code> flag does NOT release the array,
				282	and you will need to call <code>Release</code> again with a different flag
				283	eventually.
				284	</p><p>
				285	</p><p>
				286
				287
				288	</p><h2><a name="RegionCalls"> Region Calls </a></h2>
				289
				290	<p>
				291	There is an alternative to calls like <code>Get<Type>ArrayElements</code>
				292	and <code>GetStringChars</code> that may be very helpful when all you want
				293	to do is copy data in or out. Consider the following:
				294	<pre>
				295	jbyte* data = env->GetByteArrayElements(array, NULL);
				296	if (data != NULL) {
				297	memcpy(buffer, data, len);
				298	env->ReleaseByteArrayElements(array, data, JNI_ABORT);
				299	}
				300	</pre>
				301	<p>
				302	This grabs the array, copies the first <code>len</code> byte
				303	elements out of it, and then releases the array. Depending upon the VM
				304	policies the <code>Get</code> call will either pin or copy the array contents.
				305	We copy the data (for perhaps a second time), then call Release; in this case
				306	we use <code>JNI_ABORT</code> so there's no chance of a third copy.
				307	</p><p>
				308	We can accomplish the same thing with this:
				309	<pre>
				310	env->GetByteArrayRegion(array, 0, len, buffer);
				311	</pre>
				312	</p><p>
				313	This accomplishes the same thing, with several advantages:
				314	<ul>
				315	<li>Requires one JNI call instead of 3, reducing overhead.
				316	<li>Doesn't require pinning or extra data copies.
				317	<li>Reduces the risk of programmer error -- no need to match up
				318	<code>Get</code> and <code>Release</code> calls.
				319	</ul>
				320	</p><p>
				321	Similarly, you can use the <code>Set<Type>ArrayRegion</code> call
				322	to copy data into an array, and <code>GetStringRegion</code> or
				323	<code>GetStringUTFRegion</code> to copy characters out of a
				324	<code>String</code>.
				325
				326
				327	</p><h2><a name="Exceptions"> Exceptions </a></h2>
				328	<p>
				329	<strong>You may not call most JNI functions while an exception is pending.</strong>
				330	Your code is expected to notice the exception (via the function's return value,
				331	<code>ExceptionCheck()</code>, or <code>ExceptionOccurred()</code>) and return,
				332	or clear the exception and handle it.
				333	</p><p>
				334	The only JNI functions that you are allowed to call while an exception is
				335	pending are:
				336	<font size="-1"><ul>
				337	<li>DeleteGlobalRef
				338	<li>DeleteLocalRef
				339	<li>DeleteWeakGlobalRef
				340	<li>ExceptionCheck
				341	<li>ExceptionClear
				342	<li>ExceptionDescribe
				343	<li>ExceptionOccurred
				344	<li>MonitorExit
				345	<li>PopLocalFrame
				346	<li>PushLocalFrame
Elliott Hughes	b7906ed	2009-09-09 15:16:59 -0700	[diff] [blame]	347	<li>Release<PrimitiveType>ArrayElements
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	348	<li>ReleasePrimitiveArrayCritical
				349	<li>ReleaseStringChars
				350	<li>ReleaseStringCritical
				351	<li>ReleaseStringUTFChars
				352	</ul></font>
				353	</p><p>
				354	Note that exceptions thrown by interpreted code do not "leap over" native code,
				355	and C++ exceptions thrown by native code are not handled by Dalvik.
				356	The JNI <code>Throw</code> and <code>ThrowNew</code> instructions just
				357	set an exception pointer in the current thread. Upon returning to the VM from
				358	native code, the exception will be noted and handled appropriately.
				359	</p><p>
				360	Native code can "catch" an exception by calling <code>ExceptionCheck</code> or
				361	<code>ExceptionOccurred</code>, and clear it with
				362	<code>ExceptionClear</code>. As usual,
				363	discarding exceptions without handling them can lead to problems.
				364	</p><p>
				365	There are no built-in functions for manipulating the Throwable object
				366	itself, so if you want to (say) get the exception string you will need to
				367	find the Throwable class, look up the method ID for
				368	<code>getMessage "()Ljava/lang/String;"</code>, invoke it, and if the result
				369	is non-NULL use <code>GetStringUTFChars</code> to get something you can
				370	hand to printf or a LOG macro.
				371
				372	</p><p>
				373	</p><p>
				374	</p><h2><a name="Extended_checking"> Extended Checking </a></h2>
				375	<p>
				376	JNI does very little error checking. Calling <code>SetFieldInt</code>
				377	on an Object field will succeed, even if the field is marked
				378	<code>private</code> and <code>final</code>. The
				379	goal is to minimize the overhead on the assumption that, if you've written it in native code,
				380	you probably did it for performance reasons.
				381	</p><p>
				382	Some VMs support extended checking with the "<code>-Xcheck:jni</code>" flag. If the flag is set, the VM
				383	puts a different table of functions into the JavaVM and JNIEnv pointers. These functions do
				384	an extended series of checks before calling the standard implementation.
				385
				386	</p><p>
				387	Some things that may be verified:
				388	</p><p>
				389	</p>
				390	<ul>
				391	<li> Check for null pointers where not allowed.
				392	<li>
				393	<li> Verify argument type correctness (jclass is a class object,
				394	jfieldID points to field data, jstring is a java.lang.String).
				395	</li>
				396	<li> Field type correctness, e.g. don't store a HashMap in a String field.
				397	</li>
				398	<li> Check to see if an exception is pending on calls where pending exceptions are not legal.
				399	</li>
				400	<li> Check for calls to inappropriate functions between Critical get/release calls.
				401	</li>
				402	<li> Check that JNIEnv structs aren't being shared between threads.
				403
				404	</li>
				405	<li> Make sure local references aren't used outside their allowed lifespan.
				406	</li>
				407	<li> UTF-8 strings contain valid "modified UTF-8" data.
				408	</li>
				409	</ul>
				410	<p>Accessibility of methods and fields (i.e. public vs. private) is not
				411	checked.
				412	<p>
				413	The Dalvik VM supports the <code>-Xcheck:jni</code> flag. For a
				414	description of how to enable it for Android apps, see
				415	<a href="embedded-vm-control.html">Controlling the Embedded VM</a>.
				416	It's currently enabled by default in the Android emulator and on
				417	"engineering" device builds.
				418
				419	</p><p>
				420	JNI checks can be modified with the <code>-Xjniopts</code> command-line
				421	flag. Currently supported values include:
				422	</p>
				423	<blockquote><dl>
				424	<dt>forcecopy
				425	<dd>When set, any function that can return a copy of the original data
				426	(array of primitive values, UTF-16 chars) will always do so. The buffers
				427	are over-allocated and surrounded with a guard pattern to help identify
				428	code writing outside the buffer, and the contents are erased before the
				429	storage is freed to trip up code that uses the data after calling Release.
				430	<dt>warnonly
				431	<dd>By default, JNI "warnings" cause the VM to abort. With this flag
				432	it continues on.
				433	</dl></blockquote>
				434
				435
				436	</p><p>
				437	</p><h2><a name="Native_Libraries"> Native Libraries </a></h2>
				438	<p>
				439	You can load native code from shared libraries with the standard
				440	<code>System.loadLibrary()</code> call. The
				441	preferred way to get at your native code is:
				442	</p><p>
				443	</p><ul>
				444	<li> Call <code>System.loadLibrary()</code> from a static class initializer. (See the earlier example, where one is used to call nativeClassInit().) The argument is the "undecorated" library name, e.g. to load "libfubar.so" you would pass in "fubar".
				445
				446	</li>
				447	<li> Provide a native function: <code><strong>jint JNI_OnLoad(JavaVM* vm, void* reserved)</strong></code>
				448	</li>
				449	<li>In <code>JNI_OnLoad</code>, register all of your native methods. You
				450	should declare
				451	the methods "static" so the names don't take up space in the symbol table
				452	on the device.
				453	</li>
				454	</ul>
				455	<p>
				456	The <code>JNI_OnLoad</code> function should look something like this if
				457	written in C:
				458	</p><blockquote><pre>jint JNI_OnLoad(JavaVM* vm, void* reserved)
				459	{
				460	JNIEnv* env;
				461	if ((vm)->GetEnv(vm, (void*) &env, JNI_VERSION_1_4) != JNI_OK)
				462	return -1;
				463
				464	/* get class with (env)->FindClass /
				465	/* register methods with (env)->RegisterNatives /
				466
				467	return JNI_VERSION_1_4;
				468	}
				469	</pre></blockquote>
				470	</p><p>
				471	You can also call <code>System.load()</code> with the full path name of the
				472	shared library. For Android apps, you can get the full path to the
				473	application's private data storage area from the context object.
				474	</p><p>
				475	Dalvik does support "discovery" of native methods that are named in a
				476	specific way (see <a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/design.html#wp615">
				477	the JNI spec</a> for details), but this is a less desirable
				478	approach. It requires more space in the shared object symbol table,
				479	loading is slower because it requires string searches through all of the
				480	loaded shared libraries, and if a method signature is wrong you won't know
				481	about it until the first time the method is actually used.
				482	</p><p>
				483
				484
				485	</p><h2><a name="64bit"> 64-bit Considerations </a></h2>
				486
				487	<p>
				488	Android is currently expected to run on 32-bit platforms. In theory it
				489	could be built for a 64-bit system, but that is not a goal at this time.
				490	For the most part this isn't something that you will need to worry about
				491	when interacting with native code,
				492	but it becomes significant if you plan to store pointers to native
				493	structures in integer fields in an object. To support architectures
				494	that use 64-bit pointers, <strong>you need to stash your native pointers in a
				495	<code>long</code> field rather than an <code>int</code></strong>.
				496
				497
				498	</p><h2><a name="Unsupported"> Unsupported Features </a></h2>
				499	<p>All JNI 1.6 features are supported, with the following exceptions:
				500	<ul>
				501	<li><code>DefineClass</code> is not implemented. Dalvik does not use
				502	Java bytecodes or class files, so passing in binary class data
				503	doesn't work. Translation facilities may be added in a future
				504	version of the VM.</li>
				505	<li><code>NewWeakGlobalRef</code> and <code>DeleteWeakGlobalRef</code>
				506	are not implemented. The
				507	VM supports weak references, but not JNI "weak global" references.
				508	These will be supported in a future release.</li>
				509	<li><code>GetObjectRefType</code> (new in 1.6) is implemented but not fully
				510	functional -- it can't always tell the difference between "local" and
				511	"global" references.</li>
				512	</ul>
				513
				514	</p>
				515
				516	<address>Copyright © 2008 The Android Open Source Project</address>
				517
				518	</body>
				519	</html>